Synopsis:
The twinned structure of the new uranium-oxide mineral was refined from the X-ray diffraction data and was found to contain fluorine and pentavalent uranium. The presence of pentavalent uranium is indicative of the reducing conditions under which the mineral formed. Nollmotzite is the first naturally occurring uranium-oxide mineral that contains fluorine.

Abstract:
Nollmotzite (IMA2017-100), Mg[UV(UVIO2)2F3O4](H2O)4, is a new uranium oxide fluoride mineral found in the Clara mine, Black Forest Mountains, Germany. Electron microprobe analysis provided the empirical formula (Mg1.06Cu0.02)Σ1.08[UV(UVIO2)2O3.85F3.15][(H2O)3.69(OH)0.31]Σ4.00 based on three U and 15 O + F atoms per formula unit. Nollmotzite is monoclinic, space group Cm, with a = 7.1015 (12) Å, b = 11.7489 (17) Å, c = 8.1954 (14) Å, β = 98.087 (14)°, V = 676.98 (19) Å3 and Z = 2. The crystal structure [twinned by reticular merohedry; refined to R = 0.0369 with GoF = 1.09 for 1527 unique observed reflections, I > 3σ(I)] is based upon [UV(UVIO2)2F3O4]2– sheets of β-U3O8 topology and contains an interlayer with MgF2(H2O)4 octahedra. Adjacent sheets are linked through F–Mg–F bonds, as well as via hydrogen bonds. The presence of fluorine and pentavalent uranium in the structure of nollmotzite has potentially important implications for the safe disposal of nuclear waste.

NdR: The name nollmotzite honors two German mineral collectors who discovered this new mineral; the name nollmotzite combines the first four letters of their surnames: Markus Noller (born 16.05.1977) and Reinhard Motzigemba (born 14.11.1952).
The fourth mineral known to contain pentavalent uranium {the others being wyartite, shinkolobweite, richetite and dehydrated wyartite, Ca(CO3)[U5+(U6+O2)2O4(OH)](H2O)3}; the first known naturally occurring uranium oxide that contains significant fluorine.

Marco E. Ciriotti

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